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The COBRA Extended Demonstrator – Conception, Characterization, Commissioning

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The COBRA Extended Demonstrator – Conception, Characterization, Commissioning The COBRA Extended Demonstrator – Conception, Characterization, Commissioning Dissertation zur Erlangung des akademischen Grades eines Doktors der Naturwissenschaften vorgelegt von Robert Temminghoff Lehrstuhl für Experimentelle Physik IV Fakultät Physik TU Dortmund – 2019 – Der Fakultät Physik der TU Dortmund zur Erlangung des akademischen Grades eines Doktors der Naturwissenschaften vorgelegte Dissertation. 1. Gutachter: Prof. Dr. Kevin Kröninger 2. Gutachter: Prof. Dr. Wolfgang Rhode Vorsitzender der Prüfungskommission: Prof. Dr. Heinz Hövel Weiteres Mitglied der Prüfungskommission: PD Dr. Ilya Akimov Datum des Einreichens der Arbeit: 26. September 2019 Datum der mündlichen Prüfung: 06. Dezember 2019 Kurzfassung Der neutrinolose doppelte β-Zerfall ist ein hypothetischer Kernzerfall dessen Nachweis grundlegende Fragen über bislang unbekannte Eigenschaften von Neutrinos liefern und auf Physik jenseits des Standard Modells der Teilchenphysik hindeuten würde. Das CO- BRA Experiment sucht nach diesem Zerfall mit Hilfe von CdZnTe Halbleiter-Detektoren. In dieser Arbeit wird ein wesentlich verbesserter Aufbau des Experiments, genannt Extended Demonstrator (XDEM), präsentiert. Um diesen zur verwirklichen, wurden wichtige Eigenschaften der verwendeten Detektoren in Labormessungen bestimmt. Außerdem wurden Simulationen durchgeführt um Aufschluß über die Wahrschein- lichkeit einen doppelten β-Zerfall zu detektieren zu erhalten und eine Abschätzung des Hintergrundes durch andere Prozesse zu erreichen. Darauf basierend wurden Daten analysiert, welche über mehr als ein halbes Jahr am LNGS Untergrundla- bor in Italien aufgezeichnet wurden. Im Vergleich zu früheren Versionen des CO- BRA Experiments wurde der Hintergrund um mehr als einen Faktor 30 reduziert bei gleichzeitiger Steigerung der Wahrscheinlichkeit ein Signal zu messen um 50 %. Es wur- den keine Hinweise auf neutrinolosen doppelten β-Zerfall gefunden. Stattdessen wurden untere Grenzen für die Halbwertszeit der Zerfälle von 116Cd und 130Te aufgestellt, 116Cd 21 130Te 21 welche t1/2 ≥ 2:7 · 10 yr und t1/2 ≥ 8:8 · 10 yr betragen. Dies sind die stärksten Grenzen auf diese Halbwertszeiten die im Rahmen des COBRA Experiments bestimmt wurden. Abstract Neutrinoless double β-decay is a hypothetical nuclear decay whose detection would shed light on unknown properties of the neutrino and physics beyond the Standard Model of particle physics. The COBRA experiment is searching for this decay using CdZnTe semiconductor detectors. In this work, a major upgrade of the experiment is presented, called extended demonstrator (XDEM). For this, important parameters of the detectors were determined in laboratory measurements. Furthermore, simulations were used to estimate the signal efficiency and potential background from other nuclear processes. Based on this, data taken over the course of more then half a year at the LNGS underground laboratory in Italy were analyzed. Compared to earlier versions of COBRA, the background is reduced by more than a factor of 30. At the same time, the signal efficiency is improved by 50 %. No sings for neutrinoless double β-decay were detected. Instead, lower limits on the half-lives of 116Cd und 130Te were set at 116Cd 21 130Te 21 t1/2 ≥ 2:7 · 10 yr and t1/2 ≥ 8:8 · 10 yr. These are the strongest limits set on these half-lives in the context of COBRA. Contents 1 Introduction 1 2 Neutrino Physics and Double β-Decay 3 2.1 β-decays . 4 2.2 The Weak Force . 5 2.3 From First Detection to Oscillations . 6 2.4 Neutrino Masses . 9 2.5 Known Unknowns . 11 2.6 Neutrinoless Double-β Decay . 12 2.6.1 BSM-Theories Giving Rise to 0νββ-Decay . 15 2.6.2 Phase-Space Factors and Nuclear Matrix Elements . 16 2.6.3 Considerations for a 0νββ-Experiment . 18 2.6.4 Status of Searches for 0νββ-decay . 20 3 The COBRA Experiment 25 3.1 CdZnTe as Radiation Detector . 25 3.2 The COBRA Demonstrator Setup . 37 3.3 Physics with the COBRA Demonstrator . 41 4 The COBRA XDEM Upgrade and Improvements to the Demonstrator 45 4.1 Changes to the Demonstrator Setup and General Infrastructure . 46 4.2 The COBRA Extended Demonstrator . 49 4.2.1 Larger Detectors . 49 4.2.2 Read-Out Electronics . 54 4.2.3 High-Voltage Supply . 58 4.2.4 Modifications to Shielding and Infrastructure . 61 5 Large CdZnTe Detectors for the XDEM 63 5.1 Interaction Depth in CPqG Detectors . 63 Contents 5.2 Characterization of XDEM Detectors . 68 5.2.1 Working Point . 73 5.2.2 Mobility-Lifetime Product . 77 5.2.3 Efficiency . 78 5.2.4 Energy Resolution . 82 5.2.5 Current-Voltage Characteristics . 85 5.2.6 Conclusion of Characterization Measurements . 92 6 Simulations for the XDEM phase 93 6.1 Efficiency Calculations . 93 6.2 Background Estimation . 96 7 Performance of the XDEM 105 7.1 Data-Taking Conditions . 105 7.2 Linearity, Energy Resolution, Interaction Depth & Multiplicity . 107 7.3 Stability . 115 7.4 Analysis of the Low-Background Spectrum . 119 7.5 Search for 0νββ-decays . 126 7.6 Discussion and Outlook . 130 8 Conclusion 133 Acknowledgments II Bibliography XXXI A Measurement of Radioisotopes in Materials Used for XDEM XXXII B Peak Contents in Background Spectrum XXXIX C Zn Content of CdZnTe Detectors XL D PCB Design XLII List of Publications and Conference Contributions XLVII List of (Co-)Supervised Theses XLIX 1 Introduction Modern particle physics, as described by the Standard Model (SM), successfully explains a vast number of phenomena in the microscopic world. It predicts experimental results with remarkable accuracy. Still, it is empirically known that the SM is not able to explain all observed phenomena. One striking example is gravity, which is well understood through the theory of general relativity. Yet, it is unknown how gravity can be reconciled with the SM. There is also the case of Dark Matter, which is believed to make up a large fraction of matter in our universe. Dark Matter could in principle be explained viably in terms of elementary particles, but so far no direct signs of Dark Matter have been found in the laboratory. One interesting way to look for physics beyond the SM is by studying neutrinos. Neutrinos are nearly massless elementary particles which barely interact with matter. The confirmation that neutrinos have in fact mass is often seen as a hint of such physics, as neutrino masses were not predicted by the SM. The existence of massive neutrinos and potential beyond Standard Model (BSM) physics could also give rise to a nuclear process called neutrinoless double β-decay (0νββ). If this decay is indeed realized in nature, it would be incredibly rare. 0νββ-decay is associated with half-lives of at least 1026 yr. Many different approaches are employed to search for this decay. One ofthese is to use CdZnTe semiconductor crystals. These can act as source and detector for 0νββ-decay at the same time. This concept is used by the COBRA experiment. COBRA is currently still in an R&D-phase in which the viability of the idea is validated. It is operated at the Laboratori Nazionali del Gran Sasso (LNGS) under- ground laboratory in Italy. The aim of this thesis is to introduce a major upgrade of COBRA called the XDEM. The conception of the XDEM project will be presented, characterization measurements to test the concept’s realizability will be shown and the commissioning of the actual experiment will be described, culminating in an analysis of first data taken with the XDEM setup. The work will be organized as follows. First, an overview of the field of neutrino physics and 0νββ-decay in particular will be given in chapter 2. In chapter 3 it will be shown how CdZnTe detectors can be used to search for 0νββ-decay together with a 1 1 Introduction short summary of the COBRA experimental apparatus and results recently achieved with it. The XDEM upgrade will then be introduced in chapter 4. Modifications to the existing COBRA setup, which made it possible to measure events at low energies not accessible before, are also described there. Central to the upgrade are novel CdZnTe detectors which are extensively characterized in chapter 5. Simulations used to better understand the behavior of the detectors in the underground setup are shown in chapter 6. Finally, in chapter 7 data acquired over more than half a year with the XDEM setup will be presented. In chapter 8 all results will be summarized. The results presented in this work were achieved by the author in collaboration with other members of COBRA. All work conducted at the LNGS was only possible as part of a team. Much planning for this work took place a TU Dortmund together with Lucas Bodenstein-Dresler [BD18], Hannah Jansen and Jan Tebrügge. Furthermore, the scientific results could not have been accomplished by the author alone. Thisis especially true for laboratory measurements presented in chapter 5: The analysis of interaction depth relied on data taken by Katja Rohatsch at TU Dresden [Roh16]. Characterization measurements were done at TU Dortmund with Lucas Bodenstein- Dresler [BD18], who performed some of the measurement, and Inga Höfman [Hö18], who adopted the efficiency-analysis for the test setup in Dortmund. Stefan Zatschler provided some ROOT-scripts which were the basis for evaluating spectra acquired in this campaign. Pre-studies, which are not presented here, but delivered valuable experience for later laboratory work, were conducted by Marcus Albrecht [Alb16], Klaus David [Dav16], Christian Schleich [Sch16] and Jan-Hendrik Arling [Arl16]. In chapter 6 simulations are presented which required to implement an accurate description of the XDEM setup into a simulation framework. This was done in collaboration with Christian Herrmann [Her16], who was alsoinvolved in the simulation of some background sources [Her18]. Some parts of this work were already published in Refs. [Tem17; Tem16; BD+18] and presented at various conferences (see chapter D). 2 2 Neutrino Physics and Double β-Decay Neutrinos are ubiquitous elementary particles.
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